Method of making grids



Oct. 12, 1948. J. w. SKEHAN METHOD OF MAKING GRIDS Filed Jan. 24, 1947 I INVENTOR. KL 5M- ZMLBY A TTORNE'KS Patented Oct. 12, 1948 METHOD OF MAKING GRIDS Joseph W. Skehan, Stamford, Conn,

Machlett Laboratories. Incorporated,

asslgnor to Springdale, Conn, a corporation of Connecticut Application January 24, 1947, Serial No. 724,077

4 Claims.

This invention relates to grids, such as are employed in electron discharge tubes, and is concerned more particularly with a novel method for making grids for use in tubes having electrodes of the planar type.

In tubes of certain kinds for radio purposes, the anode and cathode have closely spaced opposed flat faces, and the grid, which is made of a mesh of thin wires, liesbetween the anode and cathode faces and parallel to the cathode face. The spacing between the grid and cathode is of the order of a few thousandths of an inch and must be maintained with variations not exceeding .001" in either direction, if the tubes are to function properly. As the cathode and grid are both heated to fairly high temperatures in operation, it has been difiicult heretofore to produce tubes of the construction described, in which the accuracy of the cathode-grid spacing is not destroyed by the buckling of the grid, when it is heated and expands.

The method of the invention may be used in the production of grids, which overcome the difilculties above set forth and may be used advantageously in the manufacture of electron tubes having planar electrodes and a close spacing between the grid and the cathode face. Such a grid comprises an open center member, preferably a perforated disc and made of tungsten, and a mesh of thintungsten wire affixed to one face of the disc by means of a high melting point soldering material. The strands of the grid in at least one dimension are under such tension, when the grid is cold, that the expansion of the strands resulting from heating during the operation of the tube, serves merely to relieve part of the tension and does not produce warping or buckling of the mesh. The initial spacing between the grid and cathode is, accordingly,

maintained.

For a better understanding of the invention,- reference may be made to the accompanying drawing, in which a Fig. 1 is a diagrammatic vertical sectional view of apparatus employed in the practice of the new method;

. Fig. 2 is a bottom plan view of the heating elements of the apparatus;

Fig. 3 is a plan view of the parts of which the grid is made; and

Figs. 4 and 5 are, respectively, plan and transverse sectional views of the grid.

One form of .the grid, that may be made by the method of the invention, is shown in Figs. 4 and 5 and it includes a flat perforated disc l0, prefer- 2 i ably made of tungsten, and a wire mesh ll secured to one face of the disc to extend across the entire opening therethrough. The mesh is woven of thin wires, preferably tungsten wires about .0015" in diameter with about 120 wires per inch in both dimensions. The mesh is secured to one surface of the disc by a high melting point soldering material and pure gold is preferred for the purpose. In the grid,,the wires of the mesh are under tension, when the grid is cold, and the tension is such, that the expansion of the wires, when they are heated to the temperatures, to which the grid is'raised during the operation of a tube containing the grid, is insufficient to release all the tension. The wires can, accordingly, not become slack during tube operation.

In the production of the grid described by the method of the invention, a piece if of oblong shape is cut from the mesh, so that the wires in the two dimensions lieparallel to the edges of the piece. The piece is longer and wider than the outside diameter of the disc in and it is mounted in a pair of clamps i3, which grip the ends of the piece tightly and can be moved apart to hold the strip taut. A flat ri'ng i l of the soldering material is placed on the taut strip between the clamps. the ring having an inner diameter somewhat greater and an outer diameter somewhat less thanthe corresponding diameters of the disc Ill. The

disc in is next placed on top' of the ring to lie substantially concentric therewith.

The next operation is that of heating the assembly to melt the soldering material, while maintaining the tension on the strip i2, and the heating is preferably carried on electrically by means of inner and outer filaments i5, i6, respectively. Filament i5 is a heavy coil bent to form a ring having inner and outer diameters approximately the same as those of disc l0, while filament I6 is a flat spiral coil mounted within coil Ill and lying in about the plane thereof. The coils are supported in any convenient way above the asssembiy to be heated and a bell I1 is lowered over the heating means and assembly. The bell has an inlet neck l8. leading to its upper end and a baiile is lying within the bell below the opening through the top thereof. During the heating, hydrogen or other suitable gas is introduced into the bell, so that the assembly being heated lies within a non-oxidizing atmosphere.

To heat the assembled parts, current is supplied simultaneously to both' filaments and the heating is continued, until the ring 14 of solder becomes molten and wets the mesh and the under surface of the disc. Throughout the heating, the

T 3 tension imposed on; strip H by the clamps is maintained and the clamps move apart slightly by reason or the expansion of the wires of the mesh as a result of the heating. The wires extending crosswise oi the mesh between'the clamps also expand because of the heating.

when the ring it has melted, the current supply "to filament i5 is cut 011, while. the filament l8 Monaco annulus against the exposed surface of the solder, and, while maintaining the tension on the strands, separately heating the assembled parts and the portion of the mesh exposed within the annuli, until the solder has melted. discontinuing the heating of the assem-bled parts to allow the solder,

' to cool, while continuing the heating oisaid por- When the grid is cold, the wires in. the mesh are under tension, because they have been secured to the disc while heated and expanded. The temperature, to which the wires are raised during the manufacture of the grid is higher than that to which the grid will be subjected in use,- and. ac-

cordingly, the wires of the mesh do not expand enough during use'to relieve all tension from the wires. As a consequence, the wires in the mesh remain taut at all times-and there is no buckling of the wires which would destroy the accuracy of the spacing between the'grid and 1. A method of making a grid including a rigid metallic annulus and a metallic'mesh secured to the annulus to extend across the opening therethrough, which comprises subjecting the strands of the mesh in at least one dimension to tension, placing an annulus of a high melting point solder the cathode.

against one face of the mesh, placing the rigid annulus against the exposed surface ofthe solder,

and, while maintaining the tension on the strands, heating the assembly to cause the solder to melt and wet'the mesh and rigid annulus, and then discontinuing the heating of the solder to allow it to cool, while continuing the heating of the portion of the mesh exposed within the rigid annulus, until the solder has set.

2. A method of making a grid including a rigid tion oi the mesh, and discontinuing the heating of said portion of set.

3. A method of making agrid including a rigid metallic mesh secured/to metallic annulus and a the annulus to extend across the opening therethrough, which comprises subjecting the strands of the mesh in at least one dimension to tension,

- placing an annulus'of a high melting point solderagainst one face of the mesh, placing the rigid annulus against the exposed surface of the solder, placing the assembly in anon-oxidizing'atmosphere, and, while" maintaining the tension on the strands, heating the assembly to cause the solder to melt and wet the mesh and rigid annulus, and then discontinuing the heating of the solder to allow it to cool, while continuing the heating of the portion of the mesh exposedwithin the rigid annulus, until the solder has set.

4. A method of making a grid including a rigid metallic annulus and a metallic mesh secured to the annulus to extend across the opening there-' 'through, whiclicomprises subjecting the strands of the mesh in at least one dimension to tension, placing an annulus of a high melting point solder against one face of the mesh, placing the rigid annulus against the exposed surface ,0! the solder, 5 placing the assembly in a non-oxidizing atmos-- phere, and, while maintaining the tension on the strands, separately heating the assembled parts and the portion of the mesh exposed within the annuli, until the solder has melted, discontinuing 40 the heating of the assembled parts to allow the metallic anulus and a metallic mesh secured to the annulus to extend across the openingtherethrough, which comprises subjecting the strands of the mesh in at least one dimension vto tension, placing an annulus of a high melting point solder against onei'aceoi" the mesh, placing the rigid 5 Number solder to cool, while continuing the heating of said portion of the mesh, after the solder has set.

JOSEPH'W. SKEHAN.

nnrnanncns crriin.

, The followingreferences are of record in'the file of this patent:

UNITED STATES PATENTS Name Date 2,194,551 Holman Mar. 28, 1940 i 2,402,119

Beggs no- June 18, 1946 the mesh, after the solder has M 

